1. Field of the Invention
The present invention relates to a semiconductor laser light source and, more particularly, to a semiconductor laser which uses a diamond active structure in a superluminescent diode (SLD) configuration as the gain medium to provide enough volume for high output power.
2. Description of the Prior Art
Semiconductor lasers and superluminescent diodes are intense, compact, and efficient light sources, with power density capability on the order of 107 W/cm2 generated in an active layer of thickness less than 1 xcexcm. A semiconductor laser diode is made by first growing epitaxially on a substrate an active layer sandwiched between n-doped and p-doped cladding layers having slightly lower refractive index than the active layer and forming a waveguide structure through which electric current can flow. One convenient method of making the active waveguide is by etching channels in the structure, thus creating a ridge. Light is guided in the ridge by total internal reflection because the refractive index in the channels and the clad regions is slightly lower than the effective refractive index in the active region under the ridge. Typically, the walls of the ridge are parallel and the light is guided in a zigzag manner in the ridge at a certain number of discrete angles which define the propagating modes in the structure. To each mode is associated an effective index which is determined by the zigzag angle. The lowest mode is the one for which the zigzag angle is the smallest.
FIG. 1(a) is a perspective view a typical ridge waveguide laser or Fabry-Perot laser. FIG. 1(b) is a top view illustrating the propagation characteristics. In a Fabry-Perot laser, feedback for laser oscillations is provided by reflections from the facets. One of the facets is only partially reflective so that a portion of the light is emitted. The spectrum of the light is quite narrow, as shown in FIG. 1(c), often narrower than 1 xc3x85. When the width of the ridge is within the range specified by equation (1) below, only the lowest order mode is allowed to propagate. The laser is then called single mode, and its output is a diffraction-limited beam with a Gaussian-like far-field pattern having a spatial distribution as shown in FIG. 1(d). Most applications require the laser to operate in a single mode.
In the laser of FIG. 1(a), the active waveguide is perpendicular to the facets. The same structure can be configured as a superluminescent diode (SLD) by processing the active waveguide at an angle with respect to the facets. FIGS. 2(a) and 2(b) illustrate the configuration of the same laser structure of FIG. 1(a) as an SLD. When the waveguide is not perpendicular to the facets, reflection from the facets is at an angle away from the waveguide, and only a very small fraction of the reflected light can be coupled into the waveguide. The waveguide ridge angle can be chosen to make the coupled reflected light as small as desired, as shown in FIG. 2(c). For example, the reflection is on the order of 10xe2x88x926 if the waveguide is at an angle of about 6xc2x0. In the absence of reflection, there is no feedback, and the output is a broad spectrum spontaneous emission light as shown in FIG. 2(d).
The SLD is a light source with several important functions. First, it is an optical amplifier which can be used to amplify an optical signal by several orders of magnitude. Second, it is a broad band source which is used in several applications such as a light source for fiber optic gyroscopes and low coherence imaging. Third, it can be inserted into an external cavity (a space between two reflectors) to make lasers, including tunable lasers that are free from undesirable lasing modes that would be caused by facet reflection. This third function will be exploited in this invention.
Low power (a few milliwatts) semiconductor lasers having an active length less than about 500 xcexcm and width of about 1 to 5 xcexcm are widely used in CDs, CD-ROMs, optical data storage, and optical communications. However, new applications are emerging that require power on the order of about 10 to 100 watts in a single xe2x80x9cguidedxe2x80x9d mode that can be readily coupled to a single mode fiber. Examples of such applications include light sources for projectors and laser printing. In view of their high saturation power density, such power is achievable in semiconductor laser materials by making the width of the active waveguide layer sufficiently large. For example, an active waveguide 1 mm wide and 1 xcexcm thick would have an area of 10xe2x88x925 cm2 and would generate up to 100 watts of output power. However, such power is only obtainable in multimode operation and is not obtainable in a single mode.
As noted above, the wave propagates in several modes that are characterized by discrete zigzag angles in the waveguide, the lowest mode having the smallest zigzag angle. At any given wavelength, the number of modes is determined by the refractive indexes of the waveguide core and its surrounding cladding. In order to operate in its lowest single mode, the waveguide width d cannot exceed a certain minimum value. For a waveguide with core reflective index, n1, clad reflective index n2, and wavelength xcex, the width for single mode operation is determined by equation (1):                     d        ≤                  λ                      2            ⁢                                                            n                  1                  2                                -                                  n                  2                  2                                                                                        (        1        )            
For a typical laser with n1=3.2 and n2=3.195, d is less than 3 xcexcm at 980 nm wavelength and the area for 1 xcexcm thickness is less than about 3xc3x9710xe2x88x928 cm2, so maximum output is on the order of about 0.3 watt. Thus, despite the high power capability of semiconductor laser materials, their output as single mode lasers is limited to only a fraction of a watt. Broad area lasers with about a 100 xcexcm stripe width have been made with output power of several watts, and arrays can be made with still higher power, but they are multimode devices with limited usefulness. The present invention has been developed to harness the power available in a wide stripe active region to create a high power laser (about 10 to about 100 watts) with single mode output.
The present invention is directed to a semiconductor light source comprising a plurality of superluminescent diodes (SLDs) disposed on a first conductivity type substrate having a facet, each SLD having a diamond shaped active region such that the front and rear end of each SLD ends in a taper; a plurality of channels respectively separating the SLDs; and a mode expander region disposed on at least one side of the tapers of the SLDs. The mode expander region is tapered into a waveguide extending to the facet. The mode expander may be used for moderate power, such as less than about 3 to 5 watts, and is desired for high power. In accordance with one aspect of the present invention, a mode filter, preferably a single mode optical fiber, with a mechanism for optical feedback in that mode, is coupled to the facet.